In automotive systems, it is often desirable to measure a pressure difference between two locations. For example, it is necessary to measure a pressure difference across a sharp edge orifice in a EGR (exhaust gas reflow) system. To perform these measurements electronic sensors are used to produce an electrical output indicative of the measured pressure. Electronic sensors commonly include a pressure sensing element for converting the applied pressure into an electrical signal, and a signal conditioning circuit for receiving the signal from the pressure sensing element and amplifies, offsets, and temperature compensates the signal to produce a signal dependent on the measured pressure only. These adjustments are commonly referred to as calibration and temperature compensation.
The pressure sensing element is typically a silicon, piezoresistive, bulk micromachined element. The signal conditioning circuitry that is required is similar to that shown and described in patent application Ser. No. 08/717,465.
One of the requirements of an EGR sensor is a slow response time. This response time has to be slow ( in the 100s of milliseconds) because the sensed medium exhibits a significant amount of fast pressure fluctuations. These fluctuations do not contain any important information and can be viewed as noise.
In the EGR and similar applications only an average value of the pressure is of interest. Therefore, this average value needs to be separated from the noise. The noise can be eliminated by filtering with a low corner frequency or other similar method with a slow response time system. A problem encountered in these type systems is that the sensing elements, including the piezoresistive elements, and the amplifiers are relatively fast and much faster than the required response time. In addition, the amplifiers are typically much faster than electromechanical sensors. For instance, with a silicon, piezoresistive, bulk micromachined type pressure sensing element this response time may need to be as much as 200-300 times faster than required. Therefore, the sensing element's response time must be slowed down by some means such as electronically.
This slow down has typically been done by creating an averaging circuit, or an active low pass filter, as shown in FIG. 1. The circuit 10 has a capacitor 12 placed across a resistor 14 and an input resistor 16. Resistor 14 is the gain resistor and normally is the highest value resistor in circuit 10 since the gain is large and is determined by the resistor ratio of resistors 14 and 16. Connecting capacitor 12 across the large value resistor 14 makes the capacitive value of capacitor 12 the smallest. This type of design makes for a cost effective use of parts. This conventional approach works for circuits that do not need any gain trimming, which is not the case with pressure sensor circuits.
What is needed is a circuit that allows for such a cost effective design to be utilized, but also allows the gain to be adjusted as needed. This type of a gain adjustment is necessary in circuits such as signal conditioning circuits of pressure sensors because the gain adjustment is necessary for calibration of the variable sensitivity of the sensing elements.